In oil fields particularly, better settling tanks are needed to provide time for oil to separate from produced brines and water. Environmental considerations require that the oil content of the produced water be reduced to an acceptable level, no more than 150-200 ppm (parts per million) of oil in the water discharged into rivers or streams nearby. Likewise, clean water is most important for reinjecting into walls for secondary recovery.
The conventional system comprises the utilization of a stripper tank or separator for passing the produced fluid therethrough for separation of oil from water. This fluid may include oil, water, and an emulsion of the two. This separator may use chemicals and/or heat to separate the oil and water from the oil-water emulsions. This water usually has an oil content of about 400 ppm of oil in the water.
Because this water contains about 400 ppm of oil, it is too polluted to dump into any streams or rivers, it has too much oil in it to use in an injection well for water flooding, and thus it must be stored or treated further.
A feature of this invention is the great savings resulting by providing a new settling tank that produces a cleaner water for injecting into injection wells.
The recovered oil soon pays for the cost of the settling tank:
400-ppm oil in inlet water to settling tank. PA0 -150-ppm oil in output water from settling tank. PA0 250-ppm oil removed and recovered. PA0 10,500 BPD (barrels per day) water through the settling tank. PA0 (10,500.times.250)/(1,000,000)=2.625 BPD oil recovered from settling tank. PA0 $14.00--a typical purchase price per barrel for foreign oil. PA0 Savings 1 year=2.6.times.365=949 barrels per year.times.$14.00=$13,286.00.
Likewise, the cleaner water will pass more freely through the formation sands to more easily displace oil towards the producing wells. It is shown below what the savings would be by just extending the life by one year of each of 80 oil wells feeding their waste water to the new settling tank. Obviously, more than one year extension of life would likely occur in the wells when considering that the average life of a well may be 20 years before secondary recovery, as by water flooding is initiated, as when the well production reaches a low of below 5 BPD, for example.
This average is difficult to calculate, as for example, production started in one field of oil wells at Sour Lake, Texas, in January 1903, and the wells were still producing nearly 1,600 barrels per day at the end of 1976.
The savings for each year extension of life of the wells due to the new settling tank in the above examplary field of 80 wells down to the minimum production of 5 BPD is: 80.times.5.times.365=146,000 bbl/oil.times.$14.00=$2,044,000.00.
In addition, this compact and space saving settling tank formed by the disclosed method is very attractive economically where space is expensive, such as on offshore platforms.
A new method for forming or assembling a settling tank likewise is disclosed hereinafter.